CHARLOTTE, N.C. - As President Bill Clinton gears up for his Democratic National Convention speech, it is hard not to notice that the former commander in chief's support for President Barack Obama has become one of the most closely documented facets of the campaign.

But during the past year, Clinton has also involved himself in a number of House Democratic primaries, often taking sides in Member-vs.-Member contests generated by redistricting.

On this front, Clinton has a mixed record.

The former president is credited with making the difference for some of the candidates he endorsed in competitive primaries. But there were other races where his popularity and political prowess proved ineffectual.

However, Clinton did not appear to calculate a candidate's viability when deciding whether to jump into a Democratic primary. Rather, those he endorsed had in common the quality of previously demonstrating loyalty to him and his wife, Secretary of State Hillary Rodham Clinton. Loyalty is the political quality the Clintons appreciate most, their allies say.

"He values it," close Clinton ally James Carville said in an interview. "And if you helped his wife, he will help you. . He's pretty up-front about it."

In Member-vs.-Member Democratic primaries, the effect of Clinton's endorsement was particularly acute.

Those who backed Hillary Rodham Clinton over Obama in the 2008 Democratic presidential primary found that they had an ally in Bill Clinton in 2012. Others, who went with Obama, saw that the popular ex-president actively working against them, often to devastating effect.

"I don't know if it's retribution," Carville said. "I think it's like, 'There are so many things I can do, and I chose do it for the people who've helped me.' I think it's pretty straight-up."

In several of the races, the candidates Clinton endorsed were able to overcome structural disadvantages to pull ahead on primary day.

Candidates used the Clinton endorsement in every way imaginable - highlighting the nod in radio and television advertisements, robocalls and direct mail. On occasion, the timing of his endorsement altered the trajectory of a race.

But in other instances, Clinton's endorsement amounted to nothing, with the candidates he backed losing - in some cases big. As the primary season winds to close, Clinton had an even record: four wins, four losses and one outstanding race.

Clark had the backing of Clinton and EMILY's List in her second bid for Congress earlier this summer. She and former Rep. Rick Nolan vied for the Democratic nomination to challenge Rep. Chip Cravaack (R) in a fall tossup race.

As much as the Clinton endorsement counted here, Nolan was able to rally the state's Democratic establishment behind his campaign.

Connecticut's 5th: Aug. 14LossEndorsed Candidate: Dan RobertiTies to Clinton: Worked with Clinton ally Carville; father is a Clinton donor

A political newcomer, Roberti finished a distant third in the Democratic primary, following Clinton's eleventh-hour endorsement. But the lateness of the support makes it hard to pin Roberti's third-place finish to Clinton.

Carville backed Roberti early on and did much to help him on the fundraising front, which helped give the campaign legitimacy.

In New Mexico's open 1st district race, Clinton endorsed former Albuquerque Mayor, ChÃ¡vez, who was involved in a competitive three-way Democratic primary. The endorsement came about four months before the June 5 primary, and then he did a robocall for ChÃ¡vez with about three weeks to go. But the endorsement was not enough to help ChÃ¡vez, and he finished well behind Bernalillo County Commissioner Michelle Lujan Grisham (who won) and state Sen. Eric Griego.

ChÃ¡vez began the race as the favorite and with the highest name recognition. But he didn't spend big on TV until the week before the primary.

A double-shot endorsement from both Clinton and Obama could not save Reyes from his primary challenger, former El Paso City Councilman Beto O'Rourke.

Clinton personally went to the El Paso district to campaign on behalf of Reyes and the incumbent still came up short.

Pennsylvania's 12th: April 24WinEndorsed Candidate: Rep. Mark CritzTies to Clinton: Critz's predecessor and former boss, the late Rep. John Murtha (D-Pa.), was a Clinton supporter in 2008.

The 2008 presidential campaign had a lot to do with Clinton's decision to weigh in on the Member-vs.-Member primary between Democratic Reps. Jason Altmire and Critz. Altmire stayed neutral, despite much courting from Clinton. Critz benefited from the fact that his old boss, Murtha, had endorsed Clinton in that race.

Clinton has a history of playing well in Pennsylvania, and his nod was critical to Critz's victory.

When Maryland released its redistricting map, Delaney was not the establishment favorite.

State mapmakers drew new Maryland lines with a state legislator in mind.

And then the Maryland political world was stunned when Clinton endorsed unknown millionaire businessman Delaney. Overnight, Delaney gained instant credibility, and the race became his to lose.

His rival, state Senate Majority Leader Rob Garagiola countered with an endorsement from Gov. Martin O'Malley, but the momentum was set and the damage was done.

California's 30thWinner: To be decided in the fallCandidate Supported: Rep. Brad ShermanTies to Clinton: Endorsed Clinton during the 2008 Democratic presidential primary

This is the lone outstanding race where Clinton has endorsed. After emerging from California's jungle primary as the top two, Sherman and Rep. Howard Berman are running against each other for a second time, with the winner of the Nov. 6 contest set to return to Congress next year.

Clinton has not exactly endorsed Sherman, but he wrote Sherman a "letter of support" more than a year ago.

For now, Sherman has the edge, and Bill Clinton's image has been squarely on his website for months.